1. Field of the Invention
The present invention relates generally to the fields of photoresist materials and polymer chemistry. More particularly, it concerns the production of photoresist compositions comprising polymers or co-polymers of norbornene or norbornene derivatives having acid labile groups pendant from the cyclic moieties which form a portion of the polymer backbone.
2. Description of Related Art
The use of photoresists is very important to the microfabrication of integrated circuits. The general sequence of events for resist processing includes (i) substrate preparation involving the degreasing and drying of the surface, such as a silicon chip, and perhaps the precoating with an adhesion promoter; (ii) photoresist coating which may be carried out by a wide variety of ways including spin coating on a fast-rotating turntable or spinner; (iii) prebake to remove traces of the coating solvent and release mechanical strains in the coated films; (iv) exposure with irradiation of the appropriate wavelength; (v) postexposure bake to activate the protecting groups (this step is not necessary for some photoresists, but is typically necessary for positive photoresists employing acidic photo-initiators; (vi) development which results in the removal of the unexposed or exposed photoresist for negative or positive photoresists, respectively; (vii) postbake to remove traces of the developer and anneal the polymer pattern; (viii) etching, metal patterning, or other manufacturing process; and (ix) stripping the photoresist from the surface of the substrate.
Most integrated circuits that are produced today are printed with I-line (365 nm) exposure technology. The latest generations of integrated circuits are being printed in the deep UV region (248 nm). The resolution that will be necessary for the manufacturing of future generations of integrated circuits is beyond the limits of the I-line and current deep UV lithography. For example, I-line technology has a resolution of only approximately 0.5 microns and 248 nm photoresists generally having a resolution of about 0.25 microns, where resolution refers to the ability to distinguish two neighboring elements of a pattern.
One of the approaches to enhanced imaging resolution is to decrease the wavelength of the irradiation source. According to The Semiconductor Industry Association's Technology Roadmap for Semiconductors, 193 nm lithography may be a promising candidate for achieving the desired high resolution. This approach is based on the known relationship between the resolution and the wavelength of the exposing irradiation. The minimum linewidth capable of being imaged by an optical element is proportional to the wavelength of the radiation used and inversely proportional to the numerical aperture of the optics with the proportionality constant depending on the sensitivity of the photoresist material, among other things. This relationship also suggests that increased resolution may be accomplished by increasing the numerical aperture of the optics. However, there are considerable technical problems and manufacturing expenses associated with such lenses. Additionally, the decrease in the numerical aperture adversely affects the depth of focus, which is important in high resolution lithography and depends inversely on the square of the numerical aperture.
Unfortunately, the implementation of 193 nm lithography has proven to be a major technological challenge. Even the best previously known lens materials absorb significantly at this wavelength. Although excellent 193 nm lens systems have been constructed, the damage threshold for the glass limits the exposure power that can be delivered to the wafer (Schenker et al. 1996; Schenker et al. 1994; Schenker et al 1995). Hence, the development of very high sensitivity resists is necessary to enable 193 nm imaging technology.
Photoresist materials designed for 193 nm exposure must have not only very high sensitivity, but must also be essentially transparent at the exposure wavelength and stable to reactive ion etching conditions. Unfortunately, the requirements of transparency at 193 nm and etch resistance have been generally considered to be mutually exclusive. If the photoresist is not sufficiently etch resistant then the portion of the substrate coated with the photoresist will not be protected and will be etched away as is the uncoated portion. It is known that high C/H ratio governs the sufficiency of etch resistance (Gokan et al. 1983). Additionally, conventional wisdom has held that aromaticity would provide the requisite C/H ratio since aromaticity combines double-bonds with ring structure, eliminating a significant number of hydrogen atoms from the molecule. In fact, aromatic compounds have proven quite useful in resist formulations for use with deep UV and I-line radiation.
Unfortunately, aromatics are generally known to be very strongly absorbing in the spectral region near 190 nm due to their allowed .pi.-.pi.* transitions in that region. Consequently, it has been shown that the phenolic resins typically used for I-line and deep UV (248-nm) photoresists, novolac and polyhydroxystyrene, respectively, are far too opaque at 193 nm to be used in formulating single-layer resist for use at that wavelength. Therefore, there is a strong need for photoresist compositions which are transparent in the spectral region near 193 nm and which provide adequate etch resistance.
Co-polymers of esters of methyl acrylate, methyl methacrylate, t-butyl acrylate, and acrylic acid, provide good imaging properties at 193 nm, but with very poor dry etch-resistance (Kunz et al. 1993; Allen et al. 1993; Kunz et al. 1996). Attempts to improve the dry etch resistance by the incorporation of pendant cycloaliphatic groups (Allen et al. 1996a; Allen et al. 1996b) have met with some success. However, Allen et al. have found that the increase in etch resistance resulting from the incorporation of cycloaliphatic groups comes at the expense of adequate imaging. The acrylate backbone, which serves to tether these cycloaliphatic groups, is known to depolymerize during reactive ion etching and ion-implantation. It is this depolymerization in response to exposure to radiation that is the basis for the positive tone function of poly(methyl methacrylate) resists.
Until the development of the present invention, a photoresist which possesses both the required transparency at 193 nm and an etch resistance equal to or greater than that of current I-line photoresists has not been known.